Chief Executive's Upfront
Thursday, 14 February 2019
Concrete Structures: An Assured Future
With new provisions to the concrete structures Standard, extensive concrete-based research programmes and increasing awareness of the importance of communication between designer and client, the appetite for designing buildings using concrete as the primary structural material is strong.
Rob Gaimster, Chief Executive
DESIGN POST EARTHQUAKES
It should be recognised that in the Canterbury and Kaikoura earthquakes, with some notable exceptions, most buildings, of all types of construction (and acknowledging that the commercial building stock in Christchurch was predominantly built using reinforced concrete structural systems), achieved the New Zealand Building Code life safety objectives.
Insurance policies undoubtedly played a major role in the significant number of buildings being demolished.
As it stands, any proposal for the Building Code to contain explicit requirements for buildings to be more robust and achieve post-disaster functionality is not on the immediate horizon. As such, it is up to structural designers to promote the use of “low damage systems” such as base isolation and PREcast Structural Systems (PRESSS), to increase the likelihood of buildings being operational after future earthquakes.
It is also incumbent upon structural designers to make sure their clients are aware of the design solutions that are being proposed for projects, particularly commercial building projects – is the client being made aware that they may be getting a (code compliant) ductile building which could be susceptible to damage with a high likelihood of being an economic write off after the first major earthquake the building encounters?
As the client, building developers and owners can then also help shape their destiny by making informed decisions on the options presented by their consultants. The role of insurance premiums or even insurance availability for low damage versus higher damage design solutions may also influence the client’s decision.
The earthquake performance of precast concrete floor systems in existing multi-storey buildings has been in the spotlight as a result of Wellington’s Statistics House performance during the Kaikoura earthquake – an unusually complex, multi-fault rupture with prolonged ground shaking.Some multi-storey buildings with these floor systems have been undergoing retrofit work to improve the support of their floors and the structural engineering sector is requesting more guidance on what the acceptable retrofit solutions are.
A significant three-year research programme to determine acceptable methods of retrofitting precast concrete floor systems that may pose an unacceptable risk is now underway. This programme is being led by the University of Auckland with the majority of funding for the programme coming from BRANZ. It is expected that findings from this research will be released to the industry as soon as practical during the life of the programme.
Researchers from the University of Auckland, University of Canterbury in collaboration with the New Zealand research centre QuakeCoRE and Tongji University in Shanghai, China, have also recently tested a two-storey concrete-walled building that incorporates “low damage systems” at the Tongji University laboratory facilities.
These “low-damage systems” included post-tensioned "rocking walls", devices that dissipate energy such as viscous dampers, and slotted beams. Incorporating these systems in new building designs has become increasingly popular in New Zealand in the wake of the Canterbury earthquakes as building designers and developers look at methods to increase the likelihood of buildings being usable immediately after major earthquake events.
The results from the Tongji University tests indicate that the “low damage systems” that have been implemented in building designs in Christchurch and Wellington in recent times are achieving excellent performance in terms of restricting damage under major simulated earthquake events, and therefore give increased confidence that buildings with these systems will achieve their objective of post-disaster functionality.
Amendment 3 of NZS 3101:2006 Concrete Structures (now cited in the Building Code regulatory system) contains more stringent requirements for the support details of precast concrete floor systems when they are used in ductile/flexible buildings. Following these requirements provides considerably more protection to the support conditions reducing the likelihood of the floors losing their support in a major earthquake.
Amendment 3 has also introduced Maximum Considered Earthquake (MCE) criteria. There is a strong case for saying that this new amendment now requires buildings to meet performance criteria in excess of the current Building Code settings.
A future consideration for the next amendment of the NZS 3101 is to incorporate tolerance requirements currently contained in the provisions of NZS 3109:1997 Concrete Construction. This acknowledges findings from the Statistics House investigation that recommended better alignment between the two Standards to ensure co-ordination of tolerance provisions across the design and construction process. This is particularly with respect to precast concrete components and their seating requirements.
NO CURRENT BARRIERS
With open and honest communication between structural designers and their clients around design options, along with the assurances gleaned from current research and increased robustness provisions in NZS 3101, resilient concrete structures are readily achievable, and there should be no barrier to considering concrete as the material of choice.
This is particularly the case for multi-storey buildings, recognising concrete’s inherent and beneficial properties with respect to durability, fire resistance, thermal performance and acoustic control. When done right, designing and building in concrete will provide durable solutions that stand the test of time.